An implantable medical device is implanted in a patient to monitor, among other things, electrical activity of a heart and to deliver appropriate electrical therapy, as required. Implantable medical devices (“IMDs”) include, for example, pacemakers, cardioverters, defibrillators, implantable cardioverter defibrillators (“ICD”), and the like. The electrical therapy produced by an IMD may include, for example, the application of stimulation pulses including pacing pulses, cardioverting pulses, and/or defibrillator pulses to reverse arrhythmias (e.g., tachycardias and bradycardias) or to stimulate the contraction of cardiac tissue (e.g., cardiac pacing) to return the heart to normal sinus rhythm.
Known IMDs monitor cardiac signals of a heart to determine if and when electrical therapy is needed to treat an arrhythmia. IMDs may track the rate or frequency of cardiac signal waveforms to determine if the rate of one or more waveforms of interest indicates an arrhythmia. For example, IMDs may calculate the rate at which ventricular waveforms of the heart occur. The ventricular waveforms may include QRS complexes and the rate at which the QRS complexes are detected is referred to as the ventricular heart rate. If the ventricular heart rate exceeds a predetermined threshold, the IMDs may determine that the heart is demonstrating symptoms of tachycardia. As a result, the IMDs may apply the electrical therapy to the heart to treat the detected tachycardia.
Known IMDs may, however, incorrectly calculate the rate at which QRS complexes occur. For example, the cardiac signals of some patients may include atypically large T-waves. The amplitude or size of these T-waves may be sufficiently large that known IMDs identify the T-waves as QRS complexes. As a result, the IMDs may count both a QRS complex and a T-wave in each cardiac cycle both as QRS complexes. If the IMDs count both the QRS complexes and T-waves in each cardiac cycle, the IMDs may incorrectly calculate the QRS complex as occurring twice as frequently as the QRS complex actually does occur. Consequently, the ventricular heart rate calculated by these IMDs may be twice as large as the actual ventricular heart rate. If the actual ventricular heart rate would not require the application of electrical therapy to treat tachycardia while the incorrectly calculated ventricular heart rate would require the therapy, the IMDs may unnecessarily apply electrical therapy.
Thus, a need exists for systems and methods for use with an IMD that discriminate between ventricular waveforms in order to determine a ventricular heart rate. Differentiating between the ventricular waveforms such as the QRS complexes and the T-waves may increase the accuracy in which the ventricular heart rates are calculated by IMDs and may reduce the potential for application of unnecessary electrical therapy to a non-tachycardic heart.